1. Field of the Invention
This invention is concerned with a coupling hook suitable for use on a lower steering arm of a three-point attaching device of a tractor for receiving and locking an implement pin or a bearing ball, such bearing balls being commonly mounted on such implement pins.
Such coupling hooks commonly comprise a housing defining a recess in which a pin or ball may be received, and a spring-loaded locking catch movable into a locking position thereof in which the catch has a contact point with a pin or ball in the recess to retain it in the recess. The locking catch is normally pivotal about a bearing pin supported by the housing into an unlocking position against the force of a spring so that pins or balls can be removed from or placed in the recess.
2. Description of Prior Art
Such a coupling hook is known from GB Patent Specification No. 1 533 862. In this construction, actuation levers are provided having pivot points arranged in particular positions with the objective of preventing the locking catch from moving into its unlocking position unintentionally when relative movement occurs between the implement pin or ball and the coupling hook. One of the actuation levers is provided with an arm which enables the implement pin or ball itself to move the catch into its unlocking position so that the pin or ball can enter the recess.
The disadvantage of this known design is that it is necessary to provide several actuation levers and that the retaining forces required for retaining the implement pin or ball in the recess in the locked position of the locking catch directly act on the pivot points of the actuation levers. When the implement pin or ball presses against the locking catch, the forces have to be accommodated directly by the pivot points.